1. Field of the Invention
The present invention relates to a method of controlling the output voltage of an induction generator.
2. Discussion of the Related Art
Many methods exist for controlling the output voltage of an induction generator operating over a very wide speed range. Most of these methods calculate internal generator variables, such as flux, from current and/or voltage measurements and thus suffer from sensitivity to a variation in the generator parameters. Other methods use shaft position sensors as part of the control loop and suffer from the resulting cost and reliability penalties.
Typically, methods of control have been developed for controlling the torque of an induction motor. For example, Direct Torque Control (DTC) has been previously used as a method of torque control for induction motors. One method of this type of torque control was developed in Japan and is described in a paper by Takahashi et al. that is entitled xe2x80x9cA New Quick Response and High Efficiency Strategy of an Induction Motorxe2x80x9d, Conf. Record, IEEE-IAS 1985 Ann. Meeting, pp. 495-502). Another DTC was developed independently in Germany and was described in a paper by Depenbrock entitled xe2x80x9cDirect Self Control for High Dynamic Performance of Inverter Fed AC Machinesxe2x80x9d, ETZ Archiv, Vol. 7, No. 7, 1985, pp. 211-218.
The objective of these methods was to simplify the induction motor control while improving its dynamic performance. While these objectives were generally achieved, the control also gave poor steady state characteristics.
Modifications of this method were proposed by Lascu et al. in a paper entitled xe2x80x9cA Modified Direct Torque Control (DTC) for Induction Motor Sensorless Drivexe2x80x9d, IEEE-IAS 1998 Ann. Meeting, pp. 415-422. However, even with these modifications, the control was sensitive to a change in a range of motor parameters.
DTC has been always applied to motor control but the inventors are not aware of DTC being applied to control of an induction generator. Further, sensorless schemes previously proposed for DTC were also parameter sensitive and in applications, such as automotive applications, this negates its usefulness.
The present invention seeks to provide a method of controlling an induction generator that is simpler and less expensive than previous methods.
The invention is primarily concerned with controlling an induction generator with a phase number equal to or greater than 3.
The present invention proposes to use flux sensing coils to obtain the stator flux magnitude and position rather than estimating the flux using motor equations as done in prior art. This is because using flux estimation gives results dependent on motor parameters.
One method of controlling the induction generator according to the present invention is by using DTC. DTC differs from vector control in that vector control requires current regulators, while DTC does not. In its original form, DTC only required regulation of torque and flux. In this application, the invention does not necessarily regulate torque, but rather it regulates machine flux and generator output voltage.
The present invention deals with control of induction generators and is inspired by DTC concepts, previously applied only to motor control. The features that distinguish this invention from the prior art include:
1) using DTC principles in generator control;
2) controlling the machine flux and the output voltage;
3) using flux sensing coils to obtain stator flux magnitude and position (existing DTC schemes use flux estimation which gives results dependent on motor parameters).
One application of the present invention is to an induction generator for automotive use and specifically to an induction machine automotive starter-alternator. Another application of the method of the invention is with a windmill.
The invention is also applicable to an induction machine with an electronically selectable number of poles.
One object of the present invention is to realize a minimal sensor implementation of a wide constant power speed range of a toroidally wound induction machine starter alternator (S/A), and specifically for generator mode voltage regulation.
Another object of the present invention is to use flux sensing coils to reduce the sensitivity to machine parameters and computational errors by providing a form of feedback control.
Another object of the present invention is to provide a control method that is applicable to a system where an inverter is used to control a generator where the speed is variable and is not controlled. In the case of an automotive application, the speed is dependent on the speed of the automotive engine and thus is not controlled. In the case of a windmill, the speed is dependent on the wind speed passing by the blades of the windmill that is also not controlled.
Yet another objective of this invention is to control the generator operating point and specifically the loading torque the generator exerts on the prime mover, such as an internal combustion engine or a windmill.
These and other objects of the invention can be accomplished by various methods of controlling an induction generator, as will be described. The objects of the invention can be accomplished by a method of controlling an induction generator using only flux sensing coils without requiring current sensors or position sensors. This method comprises the steps of measuring a stator flux in the generator using a plurality of flux sensing coils to determine a magnitude and position of the stator flux; measuring a DC voltage of an inverter, the inverter being operatively connected to the generator; comparing the measured stator flux magnitude with a desired flux to determine a flux error amount, the flux error amount being input to a flux regulator; determining a d-axis voltage, as the output of the flux regulator, so as to reduce the flux error amount; comparing a desired voltage with the measured DC voltage to determine a voltage error amount, the voltage error amount being input to a voltage regulator; determining a q-axis voltage, as the output of the voltage regulator, so as to reduce the voltage error amount; and transforming the d-axis voltage and the q-axis voltage to stationary reference frame voltages using the position of the stator flux.
The objects of the invention can also be accomplished by a method of controlling an induction generator using flux sensing coils and current sensors. This method comprises the steps of measuring a stator flux in the generator using a plurality of flux sensing coils to determine a magnitude and position of the stator flux; measuring a current in the generator using a plurality of current sensors; measuring a DC voltage of an inverter, the inverter being operatively connected to the generator; comparing the measured stator flux magnitude with a desired flux to determine a flux error amount, the flux error amount being input to a flux regulator; determining a desired d-axis current, as the output of the flux regulator, so as to reduce the flux error amount; comparing the desired d-axis current with the measured current to determine a d-axis current error amount, the d-axis current error amount being input to a d-axis current regulator; determining a d-axis voltage, as the output of the d-axis current regulator, so as to reduce the d-axis current error amount; comparing a desired DC voltage with the measured DC voltage to determine a voltage error amount, the voltage error amount being input to a voltage regulator; determining a q-axis voltage so as to reduce at least one of a torque error amount and a q-axis current error amount; and transforming the d-axis voltage and the q-axis voltage to stationary reference frame voltages using the position of the stator flux. Prior to determining the q-axis voltage discussed above, it is possible to determine a torque error amount and a q-axis current error amount as will be discussed below.
It is also possible to use the magnitude and position of the rotor flux instead of the magnitude and position of stator flux as rotor flux magnitude and position can be calculated from the stator flux magnitude and position.